Many schemes have been developed in the prior art for mode-locking lasers. All of the schemes function to phase lock the various longitudinal modes of the laser so that short pulses can be generated. Some schemes include an active element for varying the round trip gain in the resonator and are known as actively mode-locked lasers. Other schemes rely on passive elements. The subject invention falls in the latter category.
The subject invention includes the use of a transmissive element having an index of refraction which varies proportionally with the square of the amplitude of an electric field applied to the element. This phenomenon, generally referred to as the Kerr effect, has been studied and utilized for many years. For example, electric field variations have been used to create Kerr effect polarizers.
The electronic Kerr effect can be induced by the electric field of a light beam and in this case the response time of materials is so fast that it will manifest itself when the light beam passes through the element. There is a significant body of prior art which has reported that the Kerr effect can be used to shape and shorten pulses in a mode-locked laser. This approach is based on the fact that the bandwidth of a pulse can be expanded beyond the gain bandwidth through Kerr effect induced self phase modulation. Once the bandwidth of the pulse has been expanded, its duration can then be compressed by adding an appropriate dispersive delay line. It should be noted that this approach is used to shorten existing pulses which have been created with some other, independent, mode-locking scheme in the laser system.
An additional effect is observed when a laser beam having a nonuniform spatial intensity profile is passed through a material wherein the index of refraction varies with intensity of the electric field of the beam. More specifically, a beam having a nonuniform, two dimensional, lateral, spatial intensity variation will create a nonuniform variation in the index of refraction in the material such that an instantaneous focusing lens is created. This effect is called self-focusing and will self-vary the shape of the beam in proportion to its intensity.
Prior devices have been designed which rely upon Kerr effect self-focusing . For example, self-focusing has been used to create optical bistability. (See, "Optical Bistability Based on Self-focusing," by Bjorkholm et al, Optics Letters, Vol. 6, No. 7, July 1981). However, in most cases involving mode-locked lasers, the prior art teaches that spatial self-focusing effects should be minimized to avoid the distortion of the beam. Accordingly, the resonators of most prior art systems which employ Kerr effect elements to compress a pulse were designed to minimize the self-focusing effects. (See, "Intracavity Self-Phase Modulation and Pulse Compression in Mode-Locked Lasers," by von der Linde and Malvezzi, Applied Physics, B 37, 1-6 (1975)). In contrast, and in accordance the subject invention, the resonant cavity is configured to take advantage of the variations in the spatial profile in the beam created by self-focusing effects in order to increase the round trip gain of the laser in proportion to intensity (for intensities at or below the critical power) so that mode-locking operation can be produced.
Accordingly, it is an object of the subject invention to provide a new and improved mode-locked laser.
It is another object of the subject invention to provide a mode-locked laser which relies on self-focusing effects generated in a non-linear material.
It is a further object of the subject invention to provide a mode-locked laser which uses self-focusing effects to vary the spatial profile of the beam in response to an increase in intensity in order to increase the extraction of energy from the gain medium.
It is still another object of the subject invention to provide a mode-locked laser which uses self-focusing effects to vary the spatial profile of the beam in response to an increase in intensity in order to reduce the losses in the laser.
It is still another object of the subject invention to provide a passively mode-locked laser.
It is still a further object of the subject invention to provide a simple mode-locking mechanism.
It is still another object of the subject invention to provide a laser wherein the self-focusing which occurs in the gain medium is used to mode lock the laser.
It is still a further object of the subject invention to provide a mechanism for mode-locking a titanium-sapphire laser.
It is still another object of the subject invention to provide a mechanism for reliable initiating mode-locking behavior in a passively mode-locked laser.